1. Field Of The Invention
The present invention relates to a method for increasing the ink-receptivity of a light-sensitive planographic printing plate. More particularly, the present invention relates to a method for increasing the ink-receptivity of a light-sensitive planographic printing plate having a positive type light-sensitive layer comprising an o-quinonediazide based light-sensitive material.
2. Description Of The Prior Art
In the planographic printing method the property that water and oil are substantially immiscible is skillfully utilized, and a planographic printing plate comprises areas which are receptive to water and repellant to greasy ink, and areas which are repellent to water and are receptive to greasy ink, i.e., the former areas being the non-image areas and the latter areas being the image areas. Thus, a light-sensitive composition as used in the planographic printing plate must be repellent to water and be receptive to the greasy ink after the formation of the image, and the higher such a property is, the more easily a print of good quality can be obtained.
Hitherto a developing ink has been coated on the image surface during plate-making after exposure to improve the ink receptive properties thereof (see The Lithographic Manual, published by The Graphic Arts Technical Foundation, Inc., pages 10:26 and 10:31).
Positive type light-sensitive planographic printing plates using o-quinonediazide based light-sensitive materials are described in, for example, Japanese patent publication No. 28,403/1968, U.S. Pat. Nos. 3,046,110, 3,046,111, 3,046,112, 3,046,115, 3,046,118, 3,046,119, 3,046,120, 3,046,121, 3,046,122, 3,046,123, 3,647,443, 2,772,972, 2,767,092, 2,766,118, 2,859,112, 2,907,655, 3,061,430, 3,102,809, and 3,106,465. It is well known that the light-sensitive material contains the following structural unit as a light-sensitive group in the molecule and is decomposed upon irradiation with actinic light, thereby forming an alkali solution-soluble group (see C. Sus, Liebigs Annalen der Chemie, Vol. 556, page 65 (1944)). ##STR2##
These o-quinonediazide based light-sensitive materials have excellent sensitivity, image-strength, stability with time, and the like, but their ink receptive properties are nearly equal those of a lacquer for a planographic and intaglio plate (non-blinding lacquer) (see Chemistry of Lithography, published by The Graphic Arts Technical Foundation Inc., #401, pages 173 to 174). Thus, if a developing ink is not coated on the image areas after development, blind due to a gum solution as used in a finishing step results. The term "blind" or "blinding" as used herein means that the ink receptive property of the image is reduced by a hydrophilic coating which is coated on the plate after development to protect the hydrophilic property of the non-image areas (in general, an aqueous solution of gum arabic is widely used). Thus, as described above, the image areas must be covered with the developing ink to prevent the blinding
However, in accordance with this method, plate-making is complicated, a long period of time and a lot of labor are required for coating the developing ink, the designing of an automatic plate-making machine is made difficult, and an increase in cost results.
In using an o-quinonediazide based light-sensitive material as the light-sensitive layer of the planographic printing plate, a novolak type phenol resin is, as well known in the art, used as a binder. For example, the addition of an alkali-soluble phenol resin is described in German Pat. No. 888,204; U.S. Pat. Nos. 3,046,110, 3,046,116, 3,046,118, 3,046,123, etc. These light-sensitive materials can be developed with an alkali aqueous solution. Light-sensitive materials which can be developed with an aqueous solution are preferably used from the standpoint of cost, safety, pollution prevention, working environment, and the like. In general, a light-sensitive material is used in combination with the binder to improve the capabilities thereof, and as the binder, the alkali-soluble phenol resin as described in the above-described patents and shellac, rosin, and a styrene-maleic anhydride copolymer as described in U.S. Pat. No. 3,188,210, etc., are preferred for obtaining the advantages of development with an aqueous solution. As the phenol resin suitable for this purpose, those resins are preferred which are alcohol-soluble and are relatively easily soluble in alkali solution, and thus a phenol or cresol-formaldehyde resin is generally used. Phenol resin which are substituted with a large hydrophobic group can not be developed with an alkali aqueous solution. Thus, substituted phenol-aldehyde resins (hereinafter, those resins produced from substituted phenols and aldehydes as defined herein are referred to merely as "substituted phenol-aldehyde resins") are not suitable for development with the hitherto commercially used developers, e.g., an aqueous solution of sodium triphosphate as described in U.S. Pat. Nos. 3,046,110, 3,046,116, and 3,046,210. Planographic printing plates produced using a light-sensitive composition containing these substituted phenol-aldehyde resins can be developed with an alkali aqueous solution containing a small amount of an organic solvent, e.g., an aqueous solution prepared by adding several percent of an organic solvent such as benzyl alcohol, ethylene glycol monobutyl ether, or the like to JIS No. 3 sodium silicate.
U.S. Pat. No. 3,666,473 describes a photoresist composition prepared by adding an o-quinonediazide based light-sensitive material to a phenol-formaldehyde (novolak) resin which is insoluble in an alkali aqueous solution having a pH of about 12 or less and a phenol-formaldehyde (novolak or resol) resin which is rapidly soluble in an alkali aqueous solution having a pH of 12 or less. However, this composition is for a photoresist, and the object of this patent is to substantially reduce the exposure period. Thus, the requirement for the kind and property of the phenol-formaldehyde resin is principally limited to the solubility in the alkali aqueous solution and the requirement for the structure of each of compounds is not clear.